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  <meta name="description" content="以下系列文章基于FFmpeg 3.1.1和Xcode8。FFmpeg iOS库编译与集成FFmpeg解码流程 学习 FFmpeg ，就不得不提到一位大神，就是 雷霄骅，可惜天妒英才，在这里也先缅怀一下，同时也感谢他在视音频领域以及 FFmpeg 解析上做出的贡献。  我们先了解一下视频播放的流程，这里主要参考的是雷神的文章，[总结]视音频编解码技术零基础学习方法 。过程见下图（图片同样来自雷神的文">
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          FFmpeg解码流程
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        <h2 id="以下系列文章基于FFmpeg-3-1-1和Xcode8。"><a href="#以下系列文章基于FFmpeg-3-1-1和Xcode8。" class="headerlink" title="以下系列文章基于FFmpeg 3.1.1和Xcode8。"></a>以下系列文章基于FFmpeg 3.1.1和Xcode8。</h2><h4 id="FFmpeg-iOS库编译与集成"><a href="#FFmpeg-iOS库编译与集成" class="headerlink" title="FFmpeg iOS库编译与集成"></a><a target="_blank" rel="noopener" href="http://historyzhang.github.io/2016/09/26/FFmpeg%20iOS%E5%BA%93%E7%BC%96%E8%AF%91%E4%B8%8E%E9%9B%86%E6%88%90/">FFmpeg iOS库编译与集成</a></h4><h4 id="FFmpeg解码流程"><a href="#FFmpeg解码流程" class="headerlink" title="FFmpeg解码流程"></a><a target="_blank" rel="noopener" href="http://historyzhang.github.io/2016/09/30/FFmpeg%E8%A7%A3%E7%A0%81%E6%B5%81%E7%A8%8B/">FFmpeg解码流程</a></h4><hr>
<p>学习 <code>FFmpeg</code> ，就不得不提到一位大神，就是 <a target="_blank" rel="noopener" href="http://my.csdn.net/leixiaohua1020">雷霄骅</a>，可惜天妒英才，在这里也先缅怀一下，同时也感谢他在视音频领域以及 <code>FFmpeg</code> 解析上做出的贡献。</p>
<hr>
<p>我们先了解一下视频播放的流程，这里主要参考的是雷神的文章，<a target="_blank" rel="noopener" href="http://blog.csdn.net/leixiaohua1020/article/details/18893769">[总结]视音频编解码技术零基础学习方法</a> 。过程见下图（图片同样来自雷神的文章，红色框框是我注解的）。</p>
<span id="more"></span>

<p><img src="http://upload-images.jianshu.io/upload_images/606479-78a134f68de545b8.jpeg?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="20140201120523046.jpeg"></p>
<blockquote>
<p>播放一个互联网上的视频文件，需要经过以下几个步骤：解协议，解封装，解码视音频，视音频同步。<br>如果播放本地文件则不需要解协议，为以下几个步骤：解封装，解码视音频，视音频同步。</p>
</blockquote>
<p>关于每个步骤的含义还是去雷神的文章去看，这里就不啰嗦了。</p>
<p>本文重点讨论的是解封装、解码视频。对于音频的处理先不管。</p>
<blockquote>
<p><strong>解码</strong>的作用，就是将视频/音频压缩编码数据，解码成为非压缩的视频/音频原始数据。音频的压缩编码标准包含AAC，MP3，AC-3等等，视频的压缩编码标准则包含H.264，MPEG2，VC-1等等。解码是整个系统中最重要也是最复杂的一个环节。通过解码，压缩编码的视频数据输出成为非压缩的颜色数据，例如YUV420P，RGB等等；压缩编码的音频数据输出成为非压缩的音频抽样数据，例如PCM数据。</p>
</blockquote>
<p>好了，说了这么多理论，说点实在的。 <code>FFmpeg</code> 解码流程所需要调用的 <code>API</code> 依次为：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">开始—-&gt;</span><br><span class="line">av_register_all();</span><br><span class="line">avformat_open_input()；</span><br><span class="line">av_find_stream_info();</span><br><span class="line">av_find_best_stream();</span><br><span class="line">avcodec_find_decoder();</span><br><span class="line">while(av_read_frame()) &#123;</span><br><span class="line">    获取到packet—-&gt;</span><br><span class="line">    avcodec_send_packet();</span><br><span class="line">    avcodec_receive_frame();</span><br><span class="line">    获取到frame</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>上面的流程参考 <a target="_blank" rel="noopener" href="http://depthlove.github.io/2015/04/27/talk-about-FFmpeg-part1/">笔谈FFmpeg（一）</a>，其中有几个函数弃用了，所以我更新了一下。</p>
<p>简单的说一下更新的几个函数，其他的网上介绍的很多了，后面我也会推荐几篇文章。</p>
<ul>
<li><code>av_find_best_stream()</code>：</li>
</ul>
<p>之前用的都是这样的方法：  <code>穷举所有的流，查找其中种类为CODEC_TYPE_VIDEO</code> 。所以看别人的文章会有个 <code>while</code> 的循环。</p>
<ul>
<li><code>avcodec_send_packet();avcodec_receive_frame();</code>：</li>
</ul>
<p>之前用的是 <code>avcodec_decode_video2()</code> 。后来 <code>FFmpeg</code> 把函数拆分了。</p>
<ul>
<li>还有个需要注意的，<code>avcodec_find_decoder();</code> 步骤中所用到的也有所变动。下面是以前的用法：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">pCodecCtx = pFormatCtx-&gt;streams[videoindex]-&gt;codec;  </span><br><span class="line">pCodec = avcodec_find_decoder(pCodecCtx-&gt;codec_id);  </span><br></pre></td></tr></table></figure>
<p>下面是变动之后的用法：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">pCodecCtx = avcodec_alloc_context3(NULL);</span><br><span class="line">avcodec_parameters_to_context(pCodecCtx, pFormatCtx-&gt;streams[videoStream]-&gt;codecpar);</span><br><span class="line">pCodec = avcodec_find_decoder(pCodecCtx-&gt;codec_id);</span><br></pre></td></tr></table></figure>
<p>通过以上的步骤，获取到 <code>frame</code> 数据就是解码后的原始视频数据。后面我们的存储或者播放也都是基于这个数据的。</p>
<hr>
<h4 id="参考文章列表："><a href="#参考文章列表：" class="headerlink" title="参考文章列表："></a>参考文章列表：</h4><ul>
<li><p> <a target="_blank" rel="noopener" href="http://blog.csdn.net/leixiaohua1020/article/details/8652605">100行代码实现最简单的基于FFMPEG+SDL的视频播放器（SDL1.x）</a>  作者：<a target="_blank" rel="noopener" href="http://my.csdn.net/leixiaohua1020">雷霄骅</a></p>
</li>
<li><p> <a target="_blank" rel="noopener" href="http://depthlove.github.io/2015/04/27/talk-about-FFmpeg-part1/">笔谈FFmpeg（一）</a> 作者：<a target="_blank" rel="noopener" href="http://depthlove.github.io/">Minmin.Sun</a></p>
</li>
<li><p> <a target="_blank" rel="noopener" href="http://blog.csdn.net/ownwell/article/details/8113980">ffmpeg解码流程</a> 作者：<a target="_blank" rel="noopener" href="http://my.csdn.net/ownWell">cyning4星运</a></p>
</li>
</ul>

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